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I am planning to get two Nest Thermostats for my 2 zone HVAC system. It looks like blue C wire is already there for both thermostats, but not connected (existing thermostats run on batteries). I think I need to connect C wire to each Nest thermostat and to C on Thermostat Zone 1 and Thermostat Zone 2 at the zone control unit (see pic).

Is that going to be sufficient to power Nest thermostats or do I have to connect C wire running from the zone control unit to the gas furnace? If so, I don’t see C slot at the zone control unit (see pic), where do I connect common wire running from the furnace to the zone control unit?

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Your thermostat C terminal is right there in front of you, between G and the motor terminals

Your zone board indeed provides C terminals on the zone terminal blocks. They're between the G terminal and the motor connections for that zone, so you can just unwind those blue wires and connect them to the respective C terminals.

The zone board doesn't need C from the furnace though

However, since your zone board has its own 24VAC transformer powering it, it doesn't need a C wire from the furnace. So, you don't need to hook up the C wire from the furnace to anything.

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  • Right, I know C wires need to be connected to zone terminals for each thermostat. What I’m not sure about is C wire running from the zone control unit to the furnace. There is a set of wires along with a spare C wire on the left (cut off from the pic) which run to the furnace.
    – Cube
    Sep 18, 2022 at 20:07
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Based on the pic/schematic, you don't need to worry about the C wire coming from the furnace. That zone controller appears to have it's own transformer connected on the right (on schematic, not shown in pic though) powering it. Confirm it's a 40VA transformer since we can't see it.

Taking a quick look at the specs of that board shows it draws ~10.75VA, so that leaves more than enough power for 2 Nest stats.

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  • Yes, I can see two transformer wires on the right connected to the zone control unit, thanks!
    – Cube
    Sep 19, 2022 at 17:24

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